The basic functionality of a Global Positioning System (GPS) receiver is determining its position by computing time delays between transmission and reception of signals transmitted from a network of GPS satellites above the earth's surface, which are received by the receiver on or near the surface of the earth. The GPS satellites transmit to the receiver absolute time information associated with the satellite signal. A respective time delay resulting from signal transmission from each of the respective satellites to the receiver is multiplied by the speed of light to determine the distance from the receiver to each of the respective satellites from which data is received. This distance is known as the pseudorange. If fewer than three satellites are used to determine a position, the distance may not be precisely determinable due to an offset between an oscillator in the receiver generating a clock signal for the receiver and the timing signal to which the satellites are synchronized. The GPS satellites also transmit to the receivers satellite-positioning data, generally known as ephemeris data.
The timing signal from each satellite includes a time tag that is used by the receiver to determine when each received signal was transmitted by each respective satellite. By knowing the exact time of transmission of each of the signals, the receiver uses the ephemeris data to calculate where each satellite was when it transmitted a signal. The receiver then combines the knowledge of respective satellite positions with the computed distances to the satellites to determine the receiver's position.
Position calculations generated from satellite signals require pseudorange measurements, ephemeris data, and absolute time of transmission, from four satellites or more to determine a three dimensional position estimate of the GPS receiver's location, which includes latitude, longitude and altitude. Measurement information from three satellites is needed to determine a two dimensional position estimate of the GPS receiver's location, which includes latitude and longitude.
Other Global Navigation Satellite Systems (GNSS) operate using similar principles as GPS described above.
Existing standardised assistance data used in GPS signal acquisition is described in the 3GPP ETSI Technical Specification 04.31.) This assistance data may include the following: coarse time, date; coarse position; fine time, Doppler for satellite with respect to a stationary receiver; almanac; ephemerides; UTC model. Some manufacturers provide their own assistance data.